Hi Bobby John,
The reason why your Major Version Upgrade (MVU) attempt to 18 failed because the server itself was not in the latest minor version (17.6). We understand you’d like to expedite this due to your project requirements, hence we have performed the planned maintenance manually for the minor version to be from 17.6 to 17.7 earlier this morning after your approval. Then you tried to perform MVU to 18 and it was successful.